This invention relates to a developing device by which fluctuation of the diameter of a toner dot image after development is inhibited, and an image forming apparatus having the developing device.
Noise on an image formed a copying machine and a printer is usually evaluated by the output image after the development and fixation. When the noise is roughly classified into a microscopic noise such as the variation of the size and the shape of the dots when the image is formed by the dots screen and the fluctuation of line width when the image is formed by lines, and macroscopic noise such as the fluctuation of the latent image caused by unevenness of the photoreceptor speed, the microscopic noise is increased in each of the courses of latent image formation, development, transferring and fixation. The image after the development for visualizing the latent image is important to reduce the noise on the final image. When the diameters of the dots after the development are largely fluctuated, the noise on the final image cannot be reduced even though the degradation of the nose in the transfer and fixation processes is inhibited.
Particularly, in the case of an image forming apparatus having developing devices in tandem and an immediate transfer member, a toner image formed on a photoreceptor drum is transferred onto an image transferring medium and then the next color toner image formed on another drum is transferred onto the same image transferring-medium. In this processes, the toner image previously transferred is reversely transferred. The noise is increased since the toner image is disordered by the reversely transfer. Therefore, the fluctuation of the diameter of the dot of the toner image formed on the photoreceptor drum by the development is to be inhibited.
The object of the invention is to provide a development device and an image forming apparatus by which the fluctuation of the dot diameter of the developed image can be inhibited into the unrecognizable range and the noise on the final image can be reduced. Further object of the invention is to provide a developing device and an image forming apparatus by which the fluctuation of the diameter of the developed dot image can be inhibited so as to make narrow the fluctuation after development and the noise on the final image can be reduced.
The invention is attained by the find by the inventors that the fluctuation of the developed dots on the image carrier is reduced and the noise on the final image is lowered when the fluctuation of the diameter of the circle corresponding to the aggregate of the toner, hereinafter referred to as the circle corresponding diameter of the toner aggregate, is small so as to satisfy the following Formula 1.
The developing device according to the invention is a developing device for developing a static latent image composed of dots formed on an image carrier, in which the relation between the average of the circle corresponding diameter of the toner aggregates in the toner image on the image carrier surface formed by developing the static latent image and the value obtained by dividing the standard deviation of the circle corresponding diameter of the toner aggregate by the average circle corresponding diameter of the toner aggregates, hereinafter referred to as CV1, satisfies the following Formula 1.
0 less than Y less than 179.01xc3x97Xxe2x88x921.9031xe2x80x83xe2x80x83(1) 
X: The diameter of the circle corresponding to toner aggregate in xcexcm (X greater than 20 xcexcm)
Y: CV1=Standard deviation of the circle corresponding diameter of the toner aggregate/Average of the circle corresponding diameter of the toner aggregates X
The outline of the dot can be correctly reproduced since the each of the dots can be formed by many toner particles because the toner with a small diameter is used in the developing device. A volume average particle diameter of the toner particles is preferably from 2 to 7 xcexcm.
The volume variation coefficient of the toner is preferably not more than 22. The volume variation coefficient is a value obtained by dividing the standard deviation by the volume average particle diameter and multiplying by 100 times, hereinafter the value is referred to as CV2. In such the toner, particle diameter distribution is narrow and the charged electricity of the each toner particle is made uniform. Accordingly, the development selectivity of the toner to the static latent image on the image carrier is reduced when the image carrier is entered into the developing zone so that the uniform development can be attained.
In a histogram showing the particle diameter distribution based on the particle number in which natural logarithm ln D classified by every 0.23 is taken on the horizontal axis, wherein D is the particle diameter of the toner particle in xcexcm, and the sum M of the relative number m1 of the toner particles included in the class of the highest frequency and the relative number m2 of the toner particles included in the class of next high frequency is preferably not less than 65%. In such the toner, the diameter distribution of the toner particles is made narrow. Consequently, the occurrence of the selective development can be surly prevented by the use of such the toner.
It is preferable that the ratio of the toner particles having the foregoing shape coefficient of the toner of from 1.2 to 1.6 is not less than 60% by volume and the variation coefficient of the shape coefficient is not more than 18%.
It is also preferable to apply an alternating current voltage overlapped with a direct current voltage to a developer carrier charged in the developing device. The amounts of the toners adhered onto each of the latent image can be made uniform by applying the alternating current voltage overlapped with the direct current voltage since the toner is gone and returned between the latent image and the developing device such as a developing sleeve according to the AC frequency. The toner particle is pulled back by the AC even if the toner is excessively adhered on the latent image.
The developing device according to the invention is a developing device for developing a static latent image composed of dots formed on the image carrier. The developing device is charged in an image forming apparatus by which four color images are each formed on the image carriers by developing by the toners Y, M, C and K, respectively, and the images are successively transferred onto the same intermediate transferring member or the same recording medium set on the intermediate transferring member. In the developing device, the relation between the average of the circle corresponding diameter of the toner aggregates of the toner image on the image carrier surface formed by developing the static latent image and the value obtained by dividing the standard deviation of the diameter of the circle corresponding to the toner aggregate by the average diameter of the circle corresponding to the toner aggregates, hereinafter referred to as CV1, satisfies the following Formula.
0 less than Y less than 90.307xc3x97X1.7589xe2x80x83xe2x80x83(2) 
X: Average circle corresponding diameter of the toner aggregate in xcexcm (X greater than 20 xcexcm)
Y: CV1=Standard deviation of the circle corresponding diameter of the toner aggregate/Average diameter of the circle corresponding to the toner aggregates X
In the case of the development in the image forming apparatus having the tandem structure, it is found that when the fluctuation of the diameter of the aggregate of each of the four color toners on each of the image carriers is small so as to satisfy the foregoing Formula 2, the fluctuation of the dots on each of the image carriers is made small and the noise on the final image is reduced. Thus the invention is attained. The outline of the dot can be correctly reproduced since the each of the dots can be formed by many toner particles because the toner with a small diameter is used in the developing device. In such the case, it is preferable that the volume average particle diameter is from 2 to 6.5 xcexcm.
The volume variation coefficient of the toner is preferably not more than 20. The volume variation coefficient is a value obtained by dividing the standard deviation by the volume average particle diameter and multiplying by 100 times, hereinafter the value is referred to as CV2. In such the toner, particle diameter distribution is narrow and the charged electricity of the each toner particle is made uniform. Accordingly, the development selectivity of the toner to the static latent image on the image carrier is reduced when the image carrier is entered into the developing zone, so that the development can be carried out uniformly.
In a histogram showing the particle diameter distribution based on the particle number in which natural logarithm ln D classified by every 0.23 is taken on the horizontal axis, wherein D is the particle diameter of the toner particle in xcexcm, and the sum M of the relative number m1 of the toner particles included in the class of the highest frequency and the relative number m2 of the toner particles included in the class of next high frequency is preferably not less than 70%. In such the toner, the diameter distribution of the toner particles is made narrow. Consequently, the occurrence of the selective development can be surly prevented by the use of such the toner.
It is preferable that the ratio of the toner particles having the foregoing shape coefficient of the toner of from 1.2 to 1.6 is not less than 65% by volume and the variation coefficient of the shape coefficient is not more than 16%.
It is also preferable to apply an alternating current voltage overlapped with a direct current voltage to a developer carrier charged in the developing device. The amounts of the toners adhered onto the each latent image can be made uniform by applying the alternating current voltage overlapped with the direct current voltage since the toner goes and returns between the latent image and the developing device such as a developing sleeve according to the AC frequency. The toner particle is pulled back by the AC even if the toner is excessively adhered on the latent image.
The image forming apparatus can give a final image with a low noise by having each the foregoing developing device.